The present invention relates to the recovery of uranium from phosphoric acid, and especially to the recovery or uranium from aqueous solutions at least 5 molar in phosphoric acid containing uranium at levels of 1 to 500 mg/L.
Uranium is recovered from aqueous solutions in a variety of processes: from solutions present in the milling of conventional ores, from streams present in the reprocessing of spent fuel and from streams present in the manufacture of phosphoric acid and/or phosphates from phosphatic rock. The last of these applications differs from the other two in that concentrated phosphoric acid 5 molar or more is a strong chelating agent for uranium that retains uranium strongly in the aqueous phase and resists its removal therefrom either into an organic phase or onto a solid phase. This effect increases with increasing phosphoric acid concentration and increasing temperature.
Liquid-liquid extraction processes for removing uranium from concentrated phosphoric acid to an organic phase have been proposed utilizing a variety of organophosphorus extractants in the organic phase to effect a partitioning of the uranium between the two phases. Among the conventional extractants proposed are mono- and di-alkylphenyl esters of phosphoric acid such as mono- and di-octylphenylphosphoric acid (OPAP) (to extract tetravalent uranium) and dialkylphosphoric acids such as di-2-ethylhexylphosphoric acid (D2EHPA) (to extract hexavalent uranium). In the latter case, trialkylphosphine oxides such as trioctyl phosphine oxide (TOPO) are generally used as synergists. A third extractant suggested is a pyrophosphate ester, for extracting tetravalent uranium.
In U.S. patent application Ser. No. 096,853, filed Nov. 23, 1979, of Tunick, Largman and Sifniades, a fourth type of extractant is disclosed, being a mixture of organophosphorus compounds formed by reacting a carboxylic acid of 8-18 carbons with PCl.sub.3 in the presence of water or by reacting the corresponding acid chloride or anhydride with phosphorous acid. Two active components which are generally present in such extractants are alkane-1,1,2-triphosphonic acid and alkane-1-hydroxy-1,1-diphosphonic acids.
Removal of uranium from aqueous solutions by adsorption onto polymeric solids has been proposed for uses associated with milling of conventional ores and reprocessing of spent fuel. Polymeric solids have not been proposed which are both resistant to concentrated phosphoric acid and of sufficient attractive force with respect to uranium to overcome the chelating effect of concentrated phosphoric acid.